1. Field of the Invention
This invention relates to enhancement of mechanical properties of ceramic membranes, such as solid electrolytes and mixed ionic-electronic conducting ceramic materials. Use of many ceramic materials, particularly of perovskite structure, has been limited due to poor mechanical properties. This invention provides enhancement of mechanical properties by introduction of a uniformly distributed, high-temperature oxidation-resistant metal phase into the brittle solid electrolyte or mixed ionic-electronic conducting ceramic materials to achieve mechanically strong ceramic/metal composites operable in an oxidation atmosphere and at elevated temperatures.
2. Description of Related Art
Much prior work has centered around stabilizing and increasing oxygen ion conduction of materials such as Bi.sub.2 O.sub.3. High oxygen ion conduction compared to that of zirconia based electrolytes has been obtained using Bi.sub.2 O.sub.3 doped with Er.sub.2 O.sub.3 or Tm.sub.2 O.sub.3, M. J. Verkerk and A. J. Burggraaf, J. Electrochem. Soc., 128, No. 1, 75-82 (1981), and using Bi.sub.2 O.sub.3 doped with yttrium, P. J. Dordor, J. Tanaka and A. Watanabe, Solid State Ionics, 25, 177-181, (1987), and using Bi.sub.2 O.sub.3 doped with Yb.sub.2 O.sub.3, H. T. Cahen, T. G. M. Van Den Belt, J. H. W. De Wit and G. H. J. Broers, Solid State Ionics, 1, 411-423, (1980). Increasing oxygen ion conductivity and structural stabilization of the FCC phase of Bi.sub.2 O.sub.3 based electrolytes has been investigated resulting in fast ion conduction in Bi.sub.2 O.sub.3 doped with oxides of Y and Tb-Lu, H. T. Cahen, J. H. W. De Wit, A. Honders, G. H. J. Broers and J. P. M. Van Den Dungen, Solid State Ionics, 1, 425-440, (1980), and Bi.sub.2 O.sub.3 doped with oxides of La, Nd, Sm, Dy, Er or Yb, H. Iwahara, T. Esaka, T. Sato and T. Takahashi, J. Solid State Chem., 39, 173-180, (1981), and Bi.sub.2 O.sub.3 doped with oxides of Er and Dy, M. J. Verkerk and A. J. Burggraaf, Solid State Ionics, 3/4, 463-467, (1981). U.S. Pat. No. 5,006,494 teaches oxygen ion conductivity of Bi.sub.2 O.sub.3 in the cubic form stabilized by 10-40 mole percent of a rare earth oxide such as yttria is greatly enhanced by inclusion of up to 10 mole percent of an oxide of a cation having a valence of 4 or greater, such as zirconia, hafnia, thoria, stannic oxide, tantalum oxide, and niobium oxide.
Mixed ionic-electronic conductors have been disclosed as solid electrolyte materials and for electrocatalysis: U.S. Pat. No. 4,793,904 teaches conversion of light hydrocarbons to synthesis gas using a solid electrolyte having a conductive metal or metal oxide coating on the cathode side which is capable of reducing oxygen to oxygen ions and a conductive coating on the anode side capable of catalyzing the oxidative conversion of methane or natural gas to synthesis gas with the solid electrolyte being a high ionic conductive material, preferably yttria or calcia stabilized zirconia, while also disclosing Bi.sub.2 O.sub.3 stabilized by a lanthanide or calcium oxide; U.S. Pat. No. 4,933,054 teaches electrocatalytic oxidative conversion of saturated hydrocarbons to unsaturated hydrocarbons in an electrogenerative cell using a solid electrolyte having a conductive coating on each side, teaching the coating on the anode side may be bismuth and preferably mixtures of silver and bismuth, with the solid electrolyte being a high ionic conductive material, preferably yttria or calcia stabilized zirconia, while also disclosing Bi.sub.2 O.sub.3 stabilized by a lanthanide or calcium oxide; U.S. Pat. No. 4,802,958 teaches electrocatalytic conversion of low molecular weight hydrocarbons to higher molecular weight hydrocarbons in an electrogenerative cell using a solid electrolyte coated with a metal or metal oxide coating on each side as taught in the 4,933,054 patent, the conductive metal or metal oxide coating on the cathode side being one capable of reducing oxygen to oxygen ions and the conductive metal or metal oxide coating on the anode side being capable of catalyzing the conversion of low molecular weight hydrocarbons to higher molecular weight hydrocarbons including bismuth oxide and preferably mixtures of bismuth oxide and silver; and U.S. Pat. No. 4,812,329 teaches a coating of oxygen-ionic-electronic conducting cerium and uranium oxide undoped or, preferably, doped with zirconia, thoria, or lanthanum oxides on cermet electrodes to provide electronic conduction for solid oxide electrochemical cells European Patent Publication No. 0 399 833 teaches multiphase mixtures of an electronically conductive material and an oxygen ion conductive material and solid membranes based upon ABO.sub.3 perovskite materials, preferably containing small amounts or no bismuth.
Solid oxide electrolytes based upon metal-containing perovskite structures are known and exemplified by U.S. Pat. Nos.: 4,851,303; 5,134,042; 5,213,911; 5,244,753; 5,298,235; and 5,306,411.
Mixed ionic and electronic conducting oxidic materials based upon 25 to 98 mole percent cubic or tetragonal ZrO.sub.2, 1.5 to 15 mole percent stabilizing oxide of alkaline earth metals, yttrium and/or rare earth metals, particularly oxides of Ca, Mg, Y, and 0.5 to 50 mole percent oxide V, Nb, Ta, Cr, Mb, W and/or Ti with usual impurities are taught by U.S. Pat. No. 4,931,214 to provide high current densities, operate at lower temperatures than present materials, provide conductivity independent of oxygen pressure and are useful in oxygen concentration cells, oxygen probes, fuel cells, and electrolysis cells. U.S. Pat. No. 3,956,194 teaches mixed electronic and ionic conductors for positive electrodes of electrochemical generators which are monophased graphite material having an alkali cation of Li, Na, K, Rb, Cs, or NH.sub.4, a transition metal of Ti, V, Cr, Mn, Fe or Mo, and a non-metallic electronegative atom of O, S, F, Cl or Br.
Solid electrolytes, such as ZrO.sub.2 -based materials, and their oxygen ion transport properties are reviewed in Khandkar, A. C. and Joshi, A. V., Solid Electrolytes: Emerging Applications and Technologies, The Electrochemical Society Interface, 2, 26-33 (1993). Materials other than ZrO.sub.2 and CeO.sub.2 -materials provide better oxygen ion transport properties, such as stabilized bismuth oxides as described in U.S. Pat. No. 5,273,628; perovskites La.sub.x-1 Sr.sub.x Co.sub.1-y Fe.sub.y O.sub.3-.delta. as described in Teracka, Y., Shang, H., Furukawa, S. and Yamazoe, N., Oxygen Permeation Through Perovskite Type Oxides, Chemistry Letters, 1743-1746, (1985); and perovskites La.sub.1-x Ba.sub.x Co.sub.1-y Fe.sub.y O.sub.3-.delta. as described in U.S Pat. No. 5,240,480. Poor mechanical properties of these materials have restricted their use. The mechanical strength of yttrium stabilized bismuth oxide is usually low due to the weak bonding between the bismuth ions and oxygen ions, which limits its use as an electrolyte membrane in many electrochemical devices. Doping certain dopants into the crystal structure of the brittle materials, such as zirconia doped yttrium stabilized bismuth oxide as taught by U.S. Pat. No. 5,006,494, resulted in improved mechanical properties, but lowered oxygen ion conductivity of the material. Mixed ionic-electronic conducting perovskite materials are usually brittle, as recognized by the 5,240,480 patent which suggests a multi layer membrane with dense and porous layers to provide improved mechanical properties. Additionally, many perovskites have phase transformations between about 300.degree. and 1200.degree. C., and therefore cracking frequently occurs during formation of the perovskites by solid state sintering or it occurs later during thermal cycling.
A method for increasing the mechanical strength and ductility of brittle ceramic materials by introduction of metal nickel inclusions into Al.sub.2 O.sub.3 matrix by sintering green powder compacts of an admixture of Al.sub.2 O.sub.3 and NiO powders surrounded by a graphite powder bed to reduce NiO to Ni in situ is taught by Tuan, W. H. and Brook, R. J., Journal of the European Ceramic Society, 6, 31-37, (1990). Production of the Ni/Al.sub.z O.sub.3 composite as taught by Tuan, et al, supra, required sintering temperatures above the melting point of nickel which had a disadvantageous effect upon properties. Moreover, these techniques involve in situ formation of a metal phase under sub-atmospheric pressure at high temperatures, and, therefore, the materials can only be used for low temperature applications. If the materials are used at high temperatures, they must be kept under a reducing environment to avoid the metal phase being re-oxidized.